A read only memory (ROM) consists of an array of semiconductor devices (diodes, bipolar or field-effect transistors) which interconnect to store an array of binary data (ones or zeros). A ROM basically consists of a memory array of programmed data and a decoder to select the data located at a desired address in the memory array.
Though there are three basic types of ROMs, mask-programmable ROMs, erasable programmable ROMs (EPROMs) and field-programmable ROMs (PROMs), the focus of the present invention is on PROMs.
PROMs are typically manufactured with all switching elements present in the array, with the connection at each row-column intersection being made by means of either a fuse element or an antifuse element. In order to store data in the PROM, these elements (either the fuse or the anti-fuse, whichever are used in the design) are selectively programmed using appropriate voltage pulses supplied by a PROM programmer. Once the elements are programmed, the data is permanently stored in the memory array.
However there is a major problem with a conventionally programmed antifuse. Typically a programmed antifuse element has a resistance of several thousand ohms and cannot be reduced further without allowing a large amount of current to flow through it. Because the access transistor will limit the current to the order of a few hundred microamps, a low resistive antifuse of several hundred ohms can not be obtained through conventional methods. The larger the resistance of a programmed antifuse element in a memory cell the harder it becomes to read the data that is permanently programmed in due to the voltage drop lost across the resistive antifuse. Thus it is desirable to create the lowest resistive antifuse as possible.